Automobile air conditioners traditionally include an orifice tube or expansion valve. Because an expansion valve is essentially a fixed valve with free flow of the refrigerant, the pressure as measured on both the high and low side is a function of both the amount of refrigerant therein, as well as the ambient temperature. A suitable pressure range for the correct refrigerant fill was established many years ago, which is based on increasing the pressure as the ambient temperature rises. More recently, automobile manufacturers have started using thermostatic expansion valves with sensing bulbs that are designed to meter, or restrict, the flow of refrigerant into the evaporator, thereby providing more efficient cooling by controlling the super heating at the evaporator outlet. The control of the metering is located at the outlet of the evaporator. These automobile manufacturers use the evaporator outlet temperature, independent of the ambient temperature, to regulate the thermostatic expansion valves, which meters the refrigerant flow and sets a maximum predetermined pressure at the evaporator outlet.
For systems having a thermostatic expansion valve and sensing bulbs, as refrigerant is added during servicing, the pressure on the low side rises until it reaches this predetermined maximum level. If this pressure level is below the gauge's established correct fill range, even if the system is fully charged, adding additional refrigerant will not bring the pressure reading up to gauge's established fully charge pressure reading. As additional refrigerant is added, the excessive pressure builds up on the high side and can only be measured using a high side gauge. Because the fully charged pressure on systems using a thermostatic expansion valve and sensing bulb remains constant and the gauge's fully charged requirement rises as the ambient temperature goes up, the higher the ambient temperature the larger the gap between the system being fully charged and it showing fully charged on the low side gauge. In these cases, using only a low side gauge can potentially cause too much refrigerant to be added and result in damage to the compressor or other problems associated with overcharging.
In light of the problems advanced above, there exists a need for a device for visually indicating when the correct amount of refrigerant has been introduced to an air conditioning system and preventing the problems associated with overfilling an air conditioner with refrigerant.